A preparation technique of using an I.sub.A metal compound such as organic lithium, etc., as the polymerization catalyst can give a block copolymer of a vinylaromatic monomer and a conjugated diene monomer. This technique is accomplished by utilizing its living anion polymerization characteristic of successively polymerizing a vinylaromatic monomer and a conjugated diene monomer, and also, if necessary, performing the terminal coupling reaction. These block copolymers, when containing a relatively small amount of the vinylaromatic compound, have an elasticity at normal temperatures similar to vulcanized natural rubber or synthetic rubber, and yet can be processed at higher temperatures similarly as thermoplastic resins. For this reason, they are called thermoplastic elastomers, and have been widely used in the field of plastic modifiers, tackifiers, etc. On the other hand, when the content of the vinylaromatic compound is relatively high, thermoplastic resins excellent in transparency and impact resistance can be obtained and they have been widely utilized, primarily in the field of packaging vessels for foods. However, the trans-linkage content at the conjugated diene portion of the polymer obtained according to such a process will generally never exceed 60%, and thus no block copolymer having a crystalline trans-conjugated diene polymer block have ever been obtained.
On the other hand, conjugated diene polymers having higher trans-1,4-linkage content have been known to have been produced according to the three techniques shown below.
(1) The preparation technique of using a Ziegler catalyst containing a transition metal as the main component.
(2) The preparation technique of using an anion polymerization catalyst system comprising an alkaline earth metal compound as the main component.
(3) The preparation technique of using a catalyst system comprising a rare earth metal compound as the main component.
The first technique which comprises using a transition metal such as nickel, cobalt, titanium, vanadium, etc., as the main component has been known to effect a high degree of stereoregular polymerization of a conjugated diene monomer. However, due to low living polymerization activity and low copolymerizability with a vinylaromatic monomer, it has been difficult to produce a block copolymer of a conjugated diene monomer and a vinylaromatic monomer according to these preparation techniques.
As to the second technique, there is an example in which an organometal compound of II.sub.A metal is used as the polymerization catalyst, but an organometallic compound of II.sub.A metal other than beryllium, magnesium can be generally synthesized but only with difficulty, and their polymerization activities were also markedly low. While organometallic compounds of beryllium and magnesium can be synthesized with relative ease, polymerization activity for conjugated dienes cannot be exhibited except for special reaction conditions, and only polymers having low molecular weight could be obtained. In contrast, as the method in which a II.sub.A metal salt of an organic acid of metals such as barium and strontium is combined with other organometallic compounds, the methods have been known in which polymerization of a conjugated diene monomer is conducted by using barium-di-tert-butoxide and an organolithium (U.S. Pat. No. 3,992,561), barium-ditert -butoxide and an organomagnesium (U.S. Pat. No. 3,846,385) or an organic compound of barium or strontium, an organolithium and an organometallic compound of II.sub.B or III.sub.A metal (U.S. Pat. No. 4,092,268), etc. In these methods, using a composite catalyst containing an II.sub.A metal compound, polymers having high molecular weight can be obtained and further copolymerization of a conjugated diene monomer with a vinylaromatic monomer is possible. However, when it is desired to obtain a polymer exhibiting crystallinity with high stereoregularity having a trans-linkage content at the conjugated diene portion exceeding 80%, the polymerization temperature is required to be made lower, whereby polymerization activity becomes lower. Particularly, when it is desired to subject a conjugated diene monomer and a vinylaromatic monomer to block copolymerization, the reaction is required to be completed for each block of polymerization reaction, and therefore high living characteristic is demanded for the active terminals. Accordingly, although there is an example in which a diblock copolymer of butadiene and styrene with a relatively lower molecular weight is obtained (U.S. Pat. No. 4,355,156), there is no block copolymer obtained with a structure as intended by the present invention.
Further, as to the third technique, there has been known a polymerization catalyst comprising an organoneodymium salt of Versatic Acid and an organomagnesium (European Patent 0091287). According to this technique, although a crystalline butadiene polymer having high trans-linkage content can be obtained, polymerization activity, particularly copolymerizability with a vinylaromatic monomer such as styrene, etc., is remarkably low, and no block copolymer of a conjugated diene monomer and vinylaromatic monomer has been obtained.
In the techniques using the polymerization catalysts of the prior art as described above, problems are involved in their catalyst characteristics, and therefore no block copolymer containing a crystalline conjugated diene polymer block having a trans-linkage content at the conjugated diene portion of 80% or higher sandwiched between two or more polymer blocks of vinylaromatic compounds as purposed for by the present invention has been obtained, and no such kind of polymer has been known up to date.
Under such a situation, the present inventors have investigated intensively about the method for preparation of a copolymer containing a crystalline trans-conjugated diene polymer block and vinylaromatic compound polymer blocks and consequently developed a process for producing the same, and found that the copolymer obtained has extremely specific properties which enables the present invention to be accomplished.